The attack took place in the town of Khalis, some 65 km northeast of Baghdad, when the worshippers were taking part in the religious ritual of Ashura, said the source who declined to be named.

Ashura marks the death of Prophet Mohammed's grandson Imam Hussein, who was killed and buried in Karbala in 680 AD. It usually takes place on the 10th day of Muharam, the first month in the Islamic lunar calendar.

During the annual Ashura commemorations, mourners, generally dressed in black, took to the streets or gathered in mosques to grieve over the death of Hussein.